Texas had barely won its independence when it decided to become a part of the United States. A referendum held soon after the Battle of San Jacinto showed Texans favoring annexation by a vote of 3,277 to 93.

The annexation question became one of the most controversial issues in American politics in the late 1830s and early 1840s. The issue was not Texas but slavery. The admission of Texas to
the Union would upset the sectional balance of power in the United States Senate, just as the admission of Missouri threatened 15 years earlier. In 1838, John Quincy Adams, now a member of the House of Representatives, staged a 22-day filibuster that successfully blocked annexation. It appeared that Congress had settled the Texas question. Texas would remain an independent republic.

At this point, pro-slavery Southerners began to popularize a conspiracy theory that would eventually bring Texas into the Union as a slave state. In 1841, John Tyler, who defended slavery as a positive good, succeeded to the presidency on the death of William Henry Harrison. Tyler and his Secretary of State, John C. Calhoun, argued that Great Britain was scheming to annex Texas and transform it into a haven for runaway slaves. (In fact, British abolitionists, but not the British government, were working to convince Texas to outlaw slavery in exchange for British foreign aid). Sam Houston played along with this ploy by conducting highly visible negotiations with the British government. If the United States would not annex Texas, Houston warned, Texas would seek the support of "some other friend."

The Texas question was the major political issue in the Presidential campaign of 1844. James Polk, the Democratic candidate, ardently supported annexation. His victory encouraged Tyler to submit a resolution to Congress calling for annexation (there were not enough votes in the Senate to ratify a treaty by the required two-thirds majority; a Congressional resolution required only a simple majority). Congress narrowly approved the resolution in 1845, making Texas the 28th state.